BMI – Are we hiding our fat behind the facts?

Disclaimer: I am not a health care professional (yet :D) and therefore I do not claim that everything written below is completely accurate. I have endeavoured to be as accurate as possible and have indicated which are my own speculations and opinions. However, if you wish to make changes to your lifestyle, I would urge you to speak with a qualified health care professional and only use the below information as a guide. 

My story:

Back in April 2015, I made a decision to become healthy and loose weight. I even blogged about my journey. It was very optimistic. I lost a lot of weight in the first 6 months. 15 kilograms, to be exact. My blood pressure returned to normal. (For every kilogram of weight loss, your systolic blood pressure decreases by 2 points. Diastolic is a similar story.) I was able to run up and down the stairs without getting out of breath. I could fit into my old clothes again. In short, whatever trial and error process I did, it worked…

…for a while.

Now, I am 13 months on from my original start point. And overall I have lost 17 kilograms. Which means that in the last 7 months I’ve lost only 2 kilograms. I didn’t get it. I could see my stomach going in, my waist line reducing, my face getting more structure and yet the scales told a different story. Having despite put in more exercise and improving my eating habits even further, my weight remained stubbornly the same. I started going to the gym even harder. I changed up my exercise regime, I listened to my body – I took care. I could see the benefits really clearly. I went up from 2kg weight to 3…to 4…to 5…and I no longer needed to make two trips to the car because I could carry all the shopping in one go. I could complete my mile walks in less time. I felt physically the most fit I have ever felt. And yet my weight did not budge. And as a result, I have been stuck in the same BMI category of ‘overweight’ for the past 7 months. Something was definitely messed up. How could I be literally ‘shrinking’ in terms of fitting into clothes and yet weigh exactly the same as before? It took me ages to work it out and I think I have. But we’ll come back to me in just a moment.

Body Mass Index (BMI)

Body Mass Index (BMI) is the most commonly used screening tool for obesity. Intended for use in adults (there is a separate measure for children and toddlers) , it is a fairly straightforward concept. All is required is the weight (in kg) and height (in metres) of the individual. Then you divide the weight in kg by the height in metres (squared).

So if Bob is a 1.86m tall man who weight 95 kilograms, his BMI will be:

  • Weight in kg : 95
  • Divided by height in metres squared : 95 / 1.86^2
  • BMI = 27.5

Below are the BMI ranges:

  • Underweight:  > 18.5 and below
  • Normal: 18.5  –  24.9
  • Overweight: 25-29.9
  • Obese: 30 – 34.9
  • Morbidly obese : 35 +

This puts Bob in the overweight category. He should probably loose some weight so that his chances of diabetes Type 2, stroke, heart attack, high blood pressure and other nasty diseases all decrease.

But let me give you some more information on Bob.

  • He is a full-time builder
  • He is also member of his local rugby team and takes part in matches every weekend
  • For this reason he works out at the gym a lot so that he can develop his muscles

Would you still say that Bob is overweight? To be honest, yes, he could very well still be overweight, we haven’t actually met Bob (and he’s made up) but the more likely answer is that actually the extra weight Bob is carrying isn’t from extra fat but actually from extra muscle!

Lets now talk about Bob’s wife, Samantha.

Samantha is a professional long distance runner. She is 1.54 m and weighs 45kg. This puts her BMI at 18.9. She is at the very low end of normal weight, but should she put on more weight? Not necessarily. Being a long distance runner means that she probably has a lot of lean muscle (Slow twitch, Type 1 muscle). Whereas Bob needs a lot of strength to be a builder and play rugby (lots of fast-twitch, Type 2 muscle AND slow twitch Type 1 muscle) While Bob requires his muscles to be powerful and strong, Samantha needs her muscles to not fatigue easily. She requires endurance muscle.

And now back to me…

Recently, I had my full body analysed and measured for body fat percentage, total body weight and given ranges for what percentage my fat should fall into. (This was all done at university as part of our teaching on exercise physiology) For my height and weight, it was recommended that I be between 21 – 33% fat from total body weight.

  • My total % of body fat was 25% : well within (if not towards the lower end) of the range
  • My BMI: 29.

Do you see my conundrum? How can I be on one hand a BMI of 29, which puts me in the overweight category and simultaneously have a low-normal body fat percentage?

One answer would be that I’m like Bob. But I’m not a builder, I don’t need to have powerful muscles and of course, I do not. So it can’t be big muscle that is weighing me down.

Another answer could be that I’m a little like Samantha. My muscles might not be big in size (hypertrophy) but the lean endurance muscles might have developed. The only issue with this theory is that Type 1 endurance muscle (the kind that Samantha has more of) doesn’t really cause an increase in weight. So it is unlikely that I was developing only this type of muscle. Otherwise, weight should have decreased as the fat burned.

A few people (both professional and non-professional) speculated that perhaps it was the bones themselves that were adding to weight! And Rightly so, bones between individuals can vary in density. It follows therefore that people with denser bone will weigh more.

My verdict: it is probably a combination of all the above. It is true that I have been doing much more endurance exercise (apart from when I lifted too quickly and damaged my wrist). Consequently I must have been burning fat at the same time as developing endurance muscle (and to some extent strength muscles). Overall, causing my weight to remain the same but on the insides the composition of fat and muscle has changed.

Final thoughts:

BMI is a screening tool. By definition it means that it is not a diagnostic tool. It won’t definitely tell us if an individual is obese or not but it can give an indication whether a person is likely to be obese or not. I think using BMI in isolation is a mistake and possibly a bit misleading. It should be, in my opinion, used alongside the personal circumstances and fitness history of an individual. BMI should also be used alongside other screening tools like the full body fat analyser and waist: hip ratios.

Furthermore: somebody with a normal BMI, normal fitness can still be carrying a lot of visceral fat. This is fat around the organs and is just as dangerous as having visible fat. A lot of it is common sense too. If you’re doing no physical activity at all and eating take aways every day, likelihood is that your fat percentage (and probably BMI) are both going to be high.

I’m not entirely sure what to make of my results now. I think I’ll probably carry on the way I’ve been going. I still have a personal goal to meet. But in the mean time, at least I know that my body fat is very much in the normal range!

Thank you for reading. If you have any thoughts, please comment below!


EDIT: 11 hours after this post was initially published, I received a very helpful comment and have taken the errors pointed out and edited them. The only difference between this edited version and the previous version is that Samantha is now a long distance runner rather than a dancer. If you notice any errors or would like to give any other suggestions, please comment below.


One thought on “BMI – Are we hiding our fat behind the facts?

  1. William Roberts has a BSc (Hons) degree in Nutrition and now studies medicine. He left the following comment after reading the post. His comment is the reason for my editing 🙂 To see the original comment, please follow the link below. If you can’t see it, then I’ve posted it here too:

    Your take home point is exactly right Vitasta, it is just a screening tool. My BMI is 30 which makes me in the obese category. There is also evidence to show that a large chunk of the population who are of normal BMI have high % body fat (sedentary life means low muscle mass so their weight is normal but they are 30% fat).

    The only thing I would say is that your comparison of people with strength and endurance muscle is slightly erroneous. Type 1 endurance fibres generally adapt by improving their ability to resist fatigue, so training them won’t increase muscle weight really, any increase in muscle weight is mostly as a result of development of type 2 fibres. (I would also argue that strength is more important for a dancer than endurance).

    So if you are wondering why you still seem to be losing fat mass (weight loss is irrelevant, fat loss is what matters), but weight is staying the same it will be because you are have stronger bigger type 2 fibres and denser bones. This is probably because your workouts while aerobic in nature, are developing strength to some degree.

    Anyway just thought it might help you understand what’s going on when what’s on the scales doesn’t make sense. Congratulations on the weight loss so far, it’s really impressive!


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